A Sunken Ship is a ship that simply either did not work out or two people broke up.
A Sunken Ship happens in two scenarios. Either first, the ship was once widely supported but now nobody supports the ship, or the ship was supported and came into being, but the two people involved in the ship ran into problems and the ship became cracked at sea. Water flooded the ship and everybody on board drowned, the ship sunk, and the relationship ended. This is different from a Wrecking Ball because it didn't end on the worst possible note. Sometimes ships sink for the best, and there is no hostility involved or no major problems involved.